Its not as of right now, but if they are adding weapons with DLC, who knows? There are only 4 LMGs in the game, so I'd imagine they'd release some more.
MG36 is a beast in BF3, but BF3 goes for realism, while COD makes an "attempt" at balance. Guns don't always translate from game to game. For instance the F2000 in MW2 was a pile of garbage. It fired just as fast but had the range of a shotgun. You'll probably never see the Vector in BF3, becasue if they made it legit in COD, that's all anyone would ever use. The Vector, aka KRISS Super V, is a SMG that fires .45ACP with virtually no recoil. .45 is a massive round and would kill anything in 1-2 rounds... yet in game it feels like you're shooting .22 or .380 out of it. I see you're problem now with the LSAT. I never play TDM in BF3, I play only Rush, CQD, and Conquest. I tend to stay away from LMGs altogether on any CQC map, I generally use them on Rush for suppressing fire on a bomb site. But if I do use them, its normally the MG36 or the QBB-95. (Oddly enough I started using the QBB-LSW in black ops and like it more than the LSAT) I don't like the PKP Pecheneg, its not that its bad, but it definitely was not an upgrade over the M249 IMO. I ended up sticking with that until the QBB-95 was released.